DrIstvaan: I've not played Myth, so I don't know.
There is no diplomacy; it's more like a "generic" RTS where you have to beat your enemy to death (surrender).
However, you don't do this by building a large amount of buildings and producing many units there. You have but a few types of units, but they have numerous tactical advantages to use and disadvantages to keep in mind, which means the game has a good bit of variety, despite the relatively low number of units.